Desuperheaters are used in many industrial fluid and gas lines to reduce the temperature of superheated process fluid and gas to a desired set point temperature. For example, desuperheaters are used in power process industries to cool superheated steam. The desuperheater injects a fine spray of atomized cooling water or other fluid, referred to herein as a spraywater cloud, into the steam pipe through which the process steam is flowing. Evaporation of the water droplets in the spraywater cloud reduces the temperature of the process steam. The resulting temperature drop can be controlled by adjusting one or more control variables, such as the volume rate of injecting the cooling water and/or the temperature of the cooling water. The size of the individual droplets in the spraywater cloud and/or the pattern of the spraywater cloud can also be adjusted to control the time required for the temperature drop.
Typically, a spraywater cloud requires some minimum length or run of straight pipe downstream from the injection point to ensure substantially complete evaporation of the individual atomized water droplets. Otherwise, the spraywater cloud may condense or not completely evaporate when a bend or split in the steam pipe is encountered. This length or run of straight pipe is typically referred to as a “downstream pipe length.” A temperature sensor is also usually located at the end of the downstream pipe length to sense the resulting temperature drop of the steam.
Desuperheaters typically are one of two main functional varieties: either mechanically atomized or steam assisted. A mechanically atomized desuperheater relies solely on the mechanical flow of the cooling water through an atomizing head to atomize the cooling water in the spraywater cloud. The cooling water flows into the atomizing head, which forms the spraywater cloud and injects the spraywater cloud into the steam pipe.
A steam assisted desuperheater includes an atomizing head that combines a high velocity stream of steam, which is called atomizing steam, with a stream of cooling water to atomize the cooling water and produce the spraywater cloud. In steam assisted desuperheaters, the individual droplets in the spraywater cloud are typically smaller in size than in mechanically atomized desuperheaters and, therefore, evaporate more rapidly inside the steam pipe. Therefore, steam assisted desuperheaters may be used in applications where a shorter downstream pipe length is available.
FIG. 1 illustrates a typical steam assisted insertion style desuperheater 10. The desuperheater 10 includes an insertion tube 11 that projects radially into a steam pipe 12 that carries process steam. The insertion tube 11 disposes a single atomizing head 13 at a central region of the pipe cross-section. The atomizing head 13 is directed to inject a spraywater cloud 14 axially along an axis 19 of the pipe 12. As used herein, the term axially is used to mean that the axis of the spraywater cloud 14 is angularly aligned more closely with the axis 19 of the pipe than with a radius of the pipe, preferably within less than about 45° of the axis 19, more preferably within less than about 5-10° of the axis 19, and most preferably parallel or coaxial with the axis 19 of the pipe 12. An atomizing steam control valve 15 controls the flow of atomizing steam to the desuperheater 10. A spraywater control valve 16 controls the flow of cooling water to the desuperheater 10. The insertion tube 11 conducts each of the atomizing steam and the cooling water separately to the atomizing head 13. The atomizing head 13 mixes the atomizing steam and the cooling water and injects the resulting spraywater cloud axially into the flow stream of process steam. The body pipe 11, however, can cause eddies and vortices in the flow of process steam. These vortices can cause undesirable vibrations or have other undesirable affects on the desuperheater. Furthermore, the downstream pipe length 17 between the desuperheater 10 and a temperature sensor 18 for this type of desuperheater can be thirty feet or more, depending on many factors, which, due to space constraints in many industrial settings, can be problematic.
FIG. 2 shows a typical mechanically atomized ring style desuperheater 20 that addresses some of the constraints with the steam assisted insertion style desuperheater 10. The ring style desuperheater 20 injects one or more spraywater clouds radially into the flow of process steam, rather than axially, as with the insertion style desuperheater 10. The ring style desuperheater 20 includes a ring body 21 and one or more nozzles 22 disposed around the circumference of the ring body 21. The ring body 21 may be an axial pipe segment through which the process steam travels axially. A spraywater manifold 23 provides cooling water to the nozzles 22. The spraywater manifold 23 is formed of various pipes that connect the nozzles 22 to a source of cooling water. Each nozzle 22 has an atomizing head 24 disposed along an interior surface of the ring body 21. The atomizing head 24 injects a spraywater cloud radially into the axial flow of steam. The ring style desuperheater 20 overcomes or significantly reduces problems with vortex eddies and vibrations that may occur with the insertion style desuperheater 10 because the ring style desuperheater 20 does not have a body pipe 11. The ring style desuperheater 20 provides more flexibility for steam lines that have greater variance of steam flow characteristics because the number of nozzles 22 may be increased or decreased to provide more or less cooling spraywater into the process steam. Further, the downstream pipe length often is shorter with the ring style desuperheater 20 than with the insertion style desuperheater 10 because the nozzles 22 inject the spraywater clouds radially. Until now, however, ring style desuperheaters have been limited to being of the mechanically atomized variety.